In said parent application, there is disclosed the use of polytetrafluoroethylene (PTFE) or like polymeric material to efficiently couple flash lamp emissions into the laser medium due to its exceptionally high diffuse reflectance characteristic of better than 99%, and which increases the laser output by as much as 100%. In one embodiment, a unique technique for fabricating the cavity includes both sintering and providing an optimal packing density prior to sintering, with the sintering to take place at atmoshperic pressure to preserve the optimal packing density as closely as possible in the sintered product. This provides an optimal void percentage for optimal reflectivity. For PTFE, packing densities approaching 1.0 g/cm.sup.3 yield an opaque optimally reflective material. Moreover, in one embodiment, the unsintered particle size is maintained at less than 50 microns to obtain the machinability required for laser cavities, with the granular starting material having an impurity content of less than 10 particles per square inch so that disintegration or degradation of the laser cavity material due to pitting is prevented, with all presintering processing done in a clean room to avoid contamination of the sintered product.
In contradistinction to prior use of sintered PTFE for reflectance targets, and in contradistinction to non-optimal prior packing densitites described and referenced in said parent application (including U.S. Pat. Nos. 3,764,364 and 4,035,085 and the work of Hsia), the invention provides a diffuse highly reflective polymeric material made into a laser cavity, with the material having a greater than 99% reflectance in the visible and near IR regions of the electromagnetic spectrum. Providing such a material for a laser cavity can result in doubling the laser output. Moreover, experimentation has shown that such material has survived energy concentrations of as much as 95 joules, making it an ideal candidate for laser cavities.
In one embodiment, a unique process is used to produce laser grade cavities, in which a compressed block of polymeric material is sintered under atmospheric pressure so that critical low presintering packing densities can be maintained in the final product. The maintenance of low packing density provides for sufficient voids to produce optimal reflectivity; and this is demonstrated by the lack of translucency of the sintered product. As part of the invention, it has been found that the preferred void volume is in the range of 30% to 50% for optimal reflectance properties.
In one embodiment involving PTFE, an initial packing density of 0.856 grams per cubic centimeter results in a final density of 1.13 grams per cubic centimeter after sintering. This produces a highly reflective sintered material which is not translucent. It is a feature of the invention that sintering is performed at atmospheric pressure, which assures that the final density of the product can be precisely controlled to optimize void volume and thus reflectivity.
With atmospheric sintering, it is a finding of the invention that once cooled, an opaque white sintered polymeric material can be formed which has a nominal reflectance of greater than 99% over the wavelength range of 300-1,400 nanometers, greater than 98.5% over a range of 250-2,000 nanometers and, inter alia, a greater than 95% reflectance over the range of 250-2,500 nanometers. This is also true of unsintered product, assuming the unsintered product has an optimal packing density. This optimal packing density can be ascertained empirically for a wide variety of resins.
The class of materials contemplated for use in this invention is defined by those materials which comprise at least one fluorinated aliphatic long chain addition polymer, in turn comprised of at least one monomer having at least one fluorine atom attached to a chain carbon atom.
Polymers which fall within the above-described class of materials are well known in the art and include the various homopolymers of the above-described monomers, co-polymers of these monomers and other monomers not of the above-described class, and crosslinked polymers formed from these homopolymers and copolymers which will conform to the hereinafter described reflectance requirements. Some examples of these polymers are: polytetrafluoroethylene, polychlorotrifluoroethylene, polychlorofluoroethylene, polyvinylidene fluoride, and polyvinyl fluoride.
Some examples of monomers which may be used to make up both homopolymers and copolymers of the above-described polymers within the aforementioned class of materials are: 1.3-butadiene hexaflouride, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene, 1.1-difluoroethylene, vinyl fluoride, 1-fluoroacrylonitrile, and fluorinated acrylic-acids such as 1-fluoroacrylic acid and 2.2-difluoroacrylic acid, and tetraflouroethylene.
Examples of other monomers which may be used with the above-described monomers to form copolymers within the aforementioned class of materials are: ethylene, propylene, acrylic acid, methacrylate esters and the like.
Other possible high reflectance polymers are Dupont FEP or fluoronated ethylene propylene copolymer; Dupont PEA, perflouroalkoxy coploymer; polyisoproplyidene fluoride; polyvinyl fluoride; polyvinylidene fluoride; polychlorofluoroethylene; and other polychlorofluoroalkenes.
With respect to doping, doped laser cavities can be made in accordance with the invention of said parent application if the dopant is stable at the sintering temperature and the dopant particle size is less than or equal to that of the particulate material utilized in making the cavity. This means that for PTFE, the dopant material size is desirably less than 50 microns.
It is also a requirement in making doped cavities that adequate mixing be performed to meet uniformity specifications. Also, the dopant and the material of the subject cavity must not interact at the sintering temperature.
Such doped materials have been previously manufactured for use as wavelength calibration standards with the various designations being WCS-HO, WCS-DO and WCS-EO corresponding to holmium, dysprosium, and erbium metal salts. Moreover, rare earth oxides such as those of lanthanum, neodymium, praeseodymium, ytterbium, yttrium, sadolinium and samarium may be utilized as dopants. Previous work has shown that inorganic metal salts that are thermally stable at the sintering temperature of the material involved may be used as a dopant.
What has herefore been found is that when polymers of the type described are sintered at atmospheric pressure, an opaque polymer is produced which exhibits unexpectedly high reflectivity. The ability to control the reflectivity by sintering under atmospheric pressure means that by merely controlling the packing density prior to sintering, one can control to a substantial certainty the density after sintering. Thus, for PTFE with an optimal density of 0.856 grams per cubic centimeter, the packing density after sintering can be controlled to 1.10-1.40 g/cm.sup.3. It is noted that 99% reflectivity in the visible region of the electromagnetic spectrum requires a final sintered density of less than 1.5 g/cm.sup.3 for PTFE.
One of the other critical parameters in the laser cavity manufacturing process is the particle size of the presintered granular material. With respect to the optimal particle size, presently, the range most useful for optimum reflectance and machinability is in the 20 to 50 micron range. PTFE resins do in fact come in particle sizes greater than 350 microns. However, these particles upon sintering yield only 97% reflectance due to lack of voids. Moreover, they are not easily machined because they crumble. Such lack of machinability as well as the 2% lower reflectance at the wavelengths of interest make the 50 micron particle size desirable for PTFE granules.
In terms of manufacture, in one embodiment, the laser grade material is prepared first by blending a suitable resin to a very fine particle size. It has been found that the laser grade material desirably has an impurity content of less than 10 particles per square inch.
It has also been found that it is critical that all presintering processing be done in a clean room to prevent contamination so that the impurity level can be kept to the above-mentioned low level.
After blending the material, for PTFE, it is compressed to a presintered density of between 1.0 and 1.2 grams per cubic centimeter. Lower pressure results in a material of high reflectance but more difficult machining problem, while over compression results in a material of lower reflectance due to increased translucence in the final product. Once compressed in a mold to a block of suitable size and shape, the block is placed on a plate and is sintered at a temperature of between 360.degree. C. to 370.degree. C. for a number of hours dependent on the size of the piece to be sintered.
Once sintered, the material is cooled slowly to avoid cracking. The final shape of the product is determined by machining of the sintered block. Machining can be done using normal machine shop equipment including lathes or milling machines, with the provision that the machining equipment be very clean in that no lubricant other than water be used in the machining. It is also a requirement that the material being machined not be compressed greatly during the machining process.
Final finishing of the product is then accomplished by sanding under a stream of water to remove any grit from the material. It is extremely important that the material be kept free of all oil or solvents excluding water at all times during the process to retain its reflective properties without contamination.
The requirement for cleanliness in this procedure cannot be overstressed. Any impurity introduced into the material at any point in the process can cause major damage to the laser cavity when exposed to high intensity light, as in a flash lamp pumped laser system.
The advantages of a diffuse laser pumping cavity made in accordance with the teachings of this invention over existing ceramic cavities, metal reflectors, barium sulfate coatings and samarium filter glass cavities are as follows:
First, the subject cavities have the highest known diffuse reflectance of any diffuse laser pump cavity substrate. Therefore less radiation is lost to absorption by the cavity material and more energy is coupled into the laser medium which results in a more efficient laser. As mentioned above, increasing the reflectance for 97% to 99% increases the laser output by 100%.
Secondly, the subject cavities are the most diffuse reflectors possible, and hence are responsible for extremely uniform pumping of the laser medium which results in an optimum beam profile.
Thirdly, the subject cavities may be fabricated using standard machine shop equipment except for the above-mentioned cleanliness requirement so that cavity geometries may be prototyped easily and relatively inexpensively. The subject material is also comparable with various coupling geometries to suit user preference for special requirements.
Most importantly, the subject cavities have exceptionally long lifetimes because they are not subject to tarnishing as are diffuse silver reflectors. Moreover, the subject cavities are compatible with gas and liquid coolants and do not degrade when exposed to ultraviolet radiation, as do barium sulfate cavities.
Moreover, regardless of the use of the subject material for laser cavities, because of its high reflectivity and machinability it can be used for other applications. The subject material may be doped to produce color reflectance standards, wavelength calibration standards and grey reflectance standards, and, as later explained, it is particularly to such color standard usage that the present application is directed. Alternatively, it can be used undoped for any of a variety of situations demanding high diffuse reflectivity.
Polymeric material having 99% reflectivity can thus be obtained in machinable shapes and can be used wherever high reflectivity material is required. Note that the subject material can be used for laser cavities in an unsintered state assuming it can be encased in glass or some transparent non-degradable encapsulating medium or carrier.
As above stated, however, the concern of the present application is with the application of such novel doped fluorinated long-chain addition polymers, doped with various pigments and/or dyestuffs, to provide diffusely reflecting color standards of vastly improved characteristics later delineated.
Turning, therefore, more specifically to the background and prior art limitations of color standards, the materials now used commercially fall into three major categories, namely: a) ceramic tiles fired with a clear glaze over a colored base material; b) colored opaque glasses; and c) painted panels backed with any number of substrates including metal, paper, wood, or the like.
Each of these materials, however, has its disadvantages. Ceramic tile may, upon improper storage, "bloom" or develop a hazy film which may change the materials chromaticity. They are also specular in character due to the glaze, which may give misleading chromaticity data as the angle of viewing or illumination is varied. Ceramic tiles also are generally quite thermochromic, thermochromicity being that property of material that leads to a change in color dependent on the temperature of the material.
Opaque glass standards are also specular, with the same disadvantages of a tile. In addition, even the most opaque glasses are to some extent translucent. This translucency also can give inaccurate readings upon measurement by colorimeters. Finally, if a glass color standard becomes scratched, the scratches may render it unusable due to uneven scatter of the incident light from the surface.
Painted panel standards are by far the most available color standards in current use. These materials are available commercially, such as the Munsell color standards produced by Munsell Color Division of Kollmorgen. They may also be produced by the individual user as needed for quality control. Advantages are the wide variety of colors available, low cost, and ease of preparation and use. Disadvantages include thermochromicity, lack of enviromental stability (colors may fade upon aging or exposure to long term light), and lack of durability of the coated surface.
The invention, on the other hand, provides a new class of color standards that is thermally and chemically stable. The color of the standards is independent of viewing geometry. The standards, as before stated, are produced from doping a fluorinated long-chain addition polymer with various pigments and/or dyestuffs. As the standards are a monolithic material as opposed to being a coated material as in the previous art, they may, if damaged or marred, readily be refinished by the user to return to original color. The materials are also waterproof and environmentally stable.
They provide color standards useful in developing consistent color reproduction for manufacturers of products such as textiles, paper, pharmaceuticals, paints and inks, with a virtually unlimited palette of colors with diffuse, durable and consistent reflectance properties. Such high diffusivity, offering reflectance properties that are nearly perfectly lambertian and with color independence of viewing geometry, coupled with their reflectance consistency and reproducability, and non-thermochromic properties that obviate the need to control temperature in a laboratory setting, and their durability and washability and easily machineable properties without loss of color or surface texture, and their retaining of uniformity throughout despite exposure to harsh environments, all render them particularly ideal for such purposes, among others, as calibrating colorimeters and spectrophotometers.